Conventionally, electric power steering apparatuses are employed in each of which an electric motor is driven in accordance with a steering torque applied to a steering wheel by a driver to thereby apply a steering assist power to the steering mechanism of a vehicle. Although brush motors have been widely employed as the electric motors of the electric power steering apparatuses, in recent years brushless motors are also employed in view of the improvement of reliability and durability and the reduction of inertia etc.
FIG. 4A is a circuit diagram of a motor control circuit contained an electric power steering apparatus of a related art. In FIG. 4A, a three-phase brushless motor 90 is employed in order to generate a steering assist power to be applied to the steering mechanism of a vehicle. Further, a motor driving circuit 93 having six MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors) is employed in order to supply a three-phase driving currents (U-phase current, V-phase current and W-phase current) to the brushless motor 90.
A power supply relay 91 selectively couples a power supply to the motor control circuit. A motor relay 94 selectively couples the motor driving circuit 93 to the brushless motor 90. Each of the power supply relay 91 and the motor relay 94 is turned on when the electric power steering apparatus is operated and turned off when the electric power steering apparatus is stopped.
The six MOS-FETs contained in the motor driving circuit 93 are controlled by a PWM (Pulse Width Modulation) signal output from a drive control portion (not shown), and the motor driving circuit 93 outputs three-phase driving currents which phases differ by 2π/3 from one another and each changes in a sine wave manner. When the brushless motor 90 is driven in this manner by using the PWM signal, the current flowing into the motor driving circuit 93 from the power supply changes largely in a short time and so a current ripple appears. Thus, in order to absorb the current ripple, a capacitor 92 is provided between two power supply lines. The capacitor 92 accumulates electric charge and discharges the electric charge thus accumulated when the current flowing into the motor driving circuit 93 from the power supply is insufficient, whereby the current ripple can be absorbed.
In the electric power steering apparatus of the related art, at the time of stopping the operation thereof, all the six MOS-FETs contained in the motor driving circuit 93 are turned off and thereafter each of the power supply relay 91 and the motor relay 94 is turned off. In this case, it is necessary to discharge the electric charge accumulated in the capacitor 92. To this end, in the motor control circuit shown in FIG. 4(a), a resistor 95 acting as a discharge circuit is provided between the two power supply lines. Thus, the electric charge accumulated in the capacitor 92 is discharged via the resistor 95 after each of the power supply relay 91 and the motor relay 94 is turned off. Alternatively, a circuit formed by connecting the resistor 95 and a switch 96 in series may be provided as the discharge circuit (see FIG. 4B). In the motor control circuit shown in FIG. 4B, the switch 96 is turned on after each of the power supply relay 91 and the motor relay 94 is turned off.
The electric charge accumulated in the capacitor 92 is discharged at the time of turning the power supply off because, at the time of performing a failure inspection of the power supply relay 91 when the power supply is turned on next, the failure inspection cannot be performed correctly if the capacitor 92 still accumulates the electric charge.
The following technique is known as to the motor control circuit of the electric power steering apparatus. A patent document 1 describes that each of three signal lines for supplying a driving current to a three-phase motor is provided with a semiconductor switching element. A patent document 2 discloses an electric power steering apparatus having a motor relay.                [Patent Document 1] JP-A-2006-21645        [Patent Document 2] JP-A-2004-330877        